This invention generally relates to digital data processing systems and more specifically to communications devices for use in such systems.
One of the more important aspects of the operation of any digital data processing system is the transfer of data to and from the processing portion of such a system. There are a wide variety of input and output units which provide this transfer of information. In the subsequent discussion, these units are called "communications devices". Teletypewriters and combined cathode ray tube, or video, and input keyboard devices represent two popular types of communications devices used to communicate with a data processing system.
In many applications it is desirable to add additional features to these communications devices. For example, a combined video terminal and keyboard device provides no typed, or "hard", copy of the information that is transferred into the system or that is displayed on the video device from the system. Many such units, however, do offer a slave printer as an option to capture some or all of this information on paper. Conversely, some teletypewriters offer a video option. Both CRT and teletypewriter devices offer, as other option devices, tape memory units and even separate central processing and related units.
These various option devices can be classified as "passive" or "active" devices. The slave printer, for example, is a passive device. Generally a passive device does not control communications between the communications device (i.e., the CRT device) and the data processing system. That is, a passive device normally only responds to data from the data processing system and passes data back to the data processing system without interferring with any other operations. Alternately, the passive device may also modify the data. In the prior art, passive devices are connected to communications devices by designing both the communications device and the option device to interact with each other. Particular leads for predefined signals are properly terminated. Conductors then are tapped off these leads through a connector for the option device.
Active devices may take over control of the communications with the data processing system. They also may process the information or may communicate directly with the communications device as a separate data processing system. Prior art active devices have generally been implemented either through the use of multiplexing, or other switching, networks or by interposing the active device directly between the communications device and the data processing system.
The particular interconnections of option devices and communications devices in the prior art are restricted. Option devices are generally designed to operate with a particular communications device manufactured by the same manufacturer. If a new option device is desirable, prior options and/or the communications device may have to be redesigned. Passive and active devices generally cannot be connected to the same locations electrically in the prior art communications devices. The foregoing restrictions effectively limit the number of option devices that can connect to a particular communications device and conversely limit the number of communications devices that can accept any particular option device.
Therefore, it is an object of this invention to provide circuitry for enabling the interconnection of communications devices and option devices.
Another object of this invention is to provide circuitry for interconnecting a communications device and a variety of option devices.
Still another object of this invention is to provide interconnection circuit that enables an option device to be utilized with a variety of communications devices.
Still another object of this invention is to provide a communications device that will accept passive and active option devices without requiring any structural circuit or program changes.